Structure Sensitivity in the Electrocatalytic Properties of Pt: I . Hydrogen Adsorption on Low Index Single Crystals and the Role of Steps

Abstract

Single crystal surfaces were used to examine the structure sensitivity of hydrogen chemisorption on Pt in dilute acid electrolyte. An electrochemical cell was coupled directly with an ultrahigh vacuum system, and low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to determine the structure and composition of the electrode surfaces. Hydrogen adsorption was examined by triangular sweep cyclic voltammetry. Contributions to the multiple peaks observed in voltammograms of Pt due specifically to anions were deduced and the effects of surface geometry alone were determined. A difference in hydrogen bond energy (3.4 kcal/mole) was observed for hydrogen on the and the surface. Hydrogen was also found to be more strongly bound at (∼4 kcal/mole) at (100) steps than on the atomically flat (100) surface. Hydrogen adsorption on highly ordered surfaces could be described quantitatively by the classical model for single site adsorption on a homogeneous surface with a weak interaction between nearest neighbor pairs. This repulsive interaction lowers the coverage at 0V (NHE) of hydrogen on a perfect (111) surface to less than a monolayer. Some evidence is presented suggesting that anion adsorption is also structure sensitive and that the "third" anodic peak observed on Pt polycrystals in is due to structure‐sensitive anion adsorption.